Metal-air battery cells include an air permeable cathode and a metallic anode separated by an aqueous electrolyte. For example, in a zinc-air battery, the anode contains zinc, and during discharge, oxygen from the ambient air is converted at the cathode to hydroxide, zinc is oxidized at the anode by the hydroxide, and water and electrons are released to provide electrical energy. Metal-air batteries have a relatively high energy density because the cathode of a metal-air battery utilizes oxygen from ambient air as a reactant in the electrochemical reaction rather than a heavier material such as a metal or metallic composition. Metal-air battery cells are often arranged in multiple cell battery packs within a common housing to provide a sufficient amount of power output. The result is a relatively light-weight battery.
Both primary and secondary metal-air batteries have been developed. A rechargeable metal-air battery is recharged by applying voltage between the anode and cathode of the metal-air battery cell and reversing the electrochemical reaction. Oxygen is discharged to the atmosphere through the air permeable cathode.
Thus, it is necessary to provide a supply of oxygen to the air cathodes of the cells. Some prior systems sweep a continuous flow of new ambient air across the air cathodes at a flow rate sufficient to achieve the desired power output. Such an arrangement is shown in U.S. Pat. No. 4,913,983 which uses a fan within the battery housing to supply a flow of ambient air to a pack of metal-air battery cells.
Given the known or measurable concentration of oxygen in the ambient air and the requirement for oxygen to operate a certain metal-air battery at a certain output level, a "stoichiometric" amount of ambient air necessary for such operation can be calculated. Many air managers for metal-air batteries draw make-up ambient air into the housing to provide four to ten times the required stoichiometric amount of air.
One problem with metal-air batteries is that the ambient humidity level can cause the metal-air battery to fail. Equilibrium vapor pressure of the metal-air battery results in an equilibrium relative humidity that is typically about 45 percent. If ambient humidity is greater than the equilibrium relative humidity value for the metal-air battery, the metal-air battery will absorb water from the air through the cathode and fail due to a condition called flooding. Flooding may cause the battery to burst. If the ambient humidity is less than the equilibrium relative humidity value for the metal-air battery, the metal-air battery will release water vapor from the electrolyte through the air cathode and fail due to drying out. In most environments where a metal-air battery is used, failure occurs from drying out.
The problems caused by ambient humidity are exacerbated in air depolarized cells because the oxygen diffusion electrode(cathode) typically passes water vapor as freely as oxygen due to the similar size and polarization of gaseous water molecules. Thus, as air is supplied to such batteries on discharge, or vented on recharge (in the case of rechargeable batteries), water vapor freely passes through the cathode as well.
Therefore, the art has recognized that a humidity level in the air passing over the air cathode differing from the humidity level within the cell will create a net transfer of water into or out of the cell, and may lead to the problems outlined above. Furthermore, such problems become more serious when large quantities of new ambient air continuously flow over the cathode.
Another problem associated with supplying a metal-air cell with continuous supplies of fresh air is transfer of carbon dioxide into the cell, where it neutralizes the electrolyte, such as potassium hydroxide. In the past, carbon dioxide absorbing layers have been placed against the exterior cathode surface to trap carbon dioxide. An example of such a system is shown in U.S. Pat. No. 4,054,725.
It has previously been proposed to separate the reactant air and cooling air streams through a metal-air battery so that the flow of cooling air may be regulated without concern for such water and carbon dioxide transfer problems. This does not solve the problems caused by such gases in the reactant air itself.
U.S. Pat. No. 4,118,544 to Przybyla describes the flooding and dry out problems and discloses a primary metal-air button cell used with watches and hearing aids. The cell interposes a barrier in the path of gas communication to the air cathode. One or more passageways (for example, one hole 0.001-0.002 inch in diameter) sized to restrict gas and water vapor access to the interior of the cell are formed in the barrier and are intended to prevent excessive moisture vapor influx or egress to or from the cell. The patent postulates that a partial vacuum is created within the cell as oxygen is utilized during discharge, and that such partial vacuum draws in more air.
The goal of Przybyla appears to be merely to restrict air access to the cathode of an individual cell. While this approach may limit the amount of dry or wet ambient air available to dry out or flood the cell, limiting all components of the air would also reduce the concentration of oxygen and possibly reduce the available output power level of the cell. Alternately, the number of holes could be such that a generously ample supply of oxygen to meet power demands passes into the cell, in which case an unnecessary excess of other components would also be introduced. Przybyla does not state that any gas passes through its holes in preference to any other gas, although this may inherently occur. In any event, Przybyla teaches such small holes that it might be difficult to generate sufficient power to operate a device such as a laptop computer, even if multiple openings were formed. These small holes also are said to require special manufacturing techniques.
Furthermore, Przybyla's cell relies on passive mixing of the components of the air to move oxygen to the cathode surface. Therefore, as the cell uses more oxygen from an already oxygen-depleted gas, the critical layer of air adjacent to the cathode may tend to become oxygen deficient.
It has been proposed in French Patent No. 2,353,142 to withdraw air at least partially exhausted of oxygen from metal-air cells, mix it in varying proportions with fresh air via a three-way valve, and return the mixture to the air cathodes. One purpose of this arrangement is to vary the output of the cells by varying the oxygen content of the incoming reactant air, which is accomplished by diluting the fresh air with air at least partially exhausted of oxygen. Another purpose of this arrangement is stated to be maintenance of a constant flow of gas on the electrodes, even when the flow rate of air consumed varies as a function of the power produced, to assure a good distribution of residual carbon dioxide and partial pressure of water vapor on the surface of the electrodes. It is thereby intended to avoid localized drying or localized concentrations of carbon dioxide. The fresh air admitted in this system includes water vapor and carbon dioxide in the proportions of the outside ambient air. Transfer of water to or from the cell will occur until equilibrium is reestablished. No method is disclosed to preferentially admit particular components of the fresh air.
Thus, there has been a need for a practical air manager system which can maintain a more stable water vapor equilibrium across the air cathode of a metal-air cell while still providing new oxygen needed for operation of the cell at desired power levels. Such a system should also be adaptable to a housing surrounding a plurality of cells rather than requiring a special plenum for each air cathode in a battery.